1. Field of the Invention
The present invention relates to a voltage compensation circuit for supplying a desired voltage to portions to which the voltage is to be supplied (hereinafter referred to as voltage-supplied portions) by compensating for voltage drops which occur because of line resistances of a voltage supply line. The present invention also relates to a display apparatus, provided with such a voltage compensation circuit, for displaying an image with multiple gray scales.
2. Description of the Related Art
FIG. 13 shows the construction of a conventional liquid crystal display apparatus. The liquid crystal display apparatus includes a liquid crystal display panel 21, a plurality of data drivers 22, and a control power supply circuit 23. The liquid crystal display panel 21 includes a plurality of picture elements (not shown) arranged in a matrix. The plurality of date drivers 22 selectively supply gray-scale voltages to the respective picture elements included in the liquid crystal display panel 21. The control power supply circuit 23 supplies the gray-scale voltages to the respective data drivers 22. The data drivers 22 are not formed on the liquid crystal display panel 21. The control power supply circuit 23 is connected to the substrate 24 via a supply line 25. The data drivers 22 are interconnected with each other via printed wirings (not shown) formed on the substrate 24. The liquid crystal display panel 21 is connected to the data drivers 22 via driving terminals (not shown) of the liquid crystal display panel 21. In order to simplify the description of the construction of the conventional liquid crystal display apparatus, scanning drivers for scanning the picture elements included in the liquid crystal display panel 21 are not shown in FIG. 13.
In the conventional liquid crystal display apparatus having the above-described construction, it is possible to sufficiently reduce the line resistances of the supply line 25 and the printed wirings. Thus, the voltage drops caused by the line resistances of the supply line 25 and the printed wirings become so small that they are negligible. Accordingly, the quality of an image displayed on the liquid crystal display apparatus with multiple gray scales is generally not degraded by the drop of the gray-scale voltage applied to one end of the supply line 25 from the control power supply circuit 23.
However, in the case where the liquid crystal display panel 21 and the voltage supply line form an integrated unit, without using the substrate 24, it is impossible to make the line resistance of the voltage supply line as low as that in the above-described conventional case. As a result, the voltage drop caused by the line resistance of the voltage supply line is not negligible. In this specification, the term "a voltage supply line" is defined as a line which connects a voltage supply circuit to voltage-supplied portions.
The liquid crystal display panel 21 and the voltage supply line form an integrated unit without using the substrate 24 by the following methods.
(1) The method (COG) in which the data drivers 22 are directly connected to a substrate of the liquid crystal display panel 21 without using a tape-automated bonding (TAB) technique or the like.
(2) The method in which thin film transistors (TFTs) of polycrystalline silicon are formed in a substrate of the liquid crystal display panel 21, and also the data drivers 22 are incorporated into the substrate.
Next, referring to FIGS. 14 and 15, the voltage drop caused by the line resistance of the voltage supply line will be described in the case where the liquid crystal display panel 21 and the voltage supply line form an integrated unit.
FIG. 14 shows the distribution of the line resistance of the voltage supply line. In general, a line resistance is a distributed constant circuit, but it can be approximated by using a plurality of concentrated constants. In FIG. 14, the line resistance of the voltage supply line 11 shown in FIG. 14 is represented by 2n concentrated constants r.sub.1 to r.sub.2n. Each of the concentrated constants r.sub.1 to r.sub.2n has a value r. It is assumed that a gray-scale voltage V is applied to one end of the voltage supply line 11, and a current i flows through the voltage supply line 11 in the direction indicated by the arrow in FIG. 14. In this case, the voltage drop at a point P.sub.s, which is closest to the source of the gray-scale voltage V, is 0. However, the voltage drop from the point P.sub.s to a point P.sub.m, which is positioned between the concentrated constants r.sub.n and r.sub.n+1, is nri. The voltage drop from the point P.sub.s to a point P.sub.e which is positioned at the other end of the voltage supply line 11 is 2nri.
FIG. 15 shows voltages at respective points on the voltage supply line 11. The voltage V.sub.s at the point P.sub.s is equal to the gray-scale voltage V. On the other hand, the voltage V.sub.m at the point P.sub.m is lower than the gray-scale voltage V by an amount corresponding to the voltage drop (nri). The voltage V.sub.e at the point P.sub.e is lower than the gray-scale voltage V by an amount corresponding to the voltage drop (2nri). The voltage drops at the respective points on the voltage supply line 11 also cause voltage drops in the data drivers 22 which are connected to the respective points on the voltage supply line 11. This results in potential difference between the gray-scale voltage output from a data driver 22 which is closer to the source of the gray-scale voltage V and the gray-scale voltage output from a data driver 22 which is remoter from the source of the gray-scale voltage V. This causes problems in that the resulting image is displayed with various non-uniform gray scales.